The present invention relates to the coating of thin substrates by deposition under vacuum. More particularly, the field of the invention is metallization of semiconductor wafers, and apparatus for effecting such metallization of wafers individually, and on a serial, continuous basis. Semiconductor wafer fabrication techniques have evolved rapidly over the past decade. Individual microcircuit devices have become progressively smaller, thereby increasing the number of such devices that can be put onto a wafer of a given size. Additionally, wafers of larger diameter are coming into use. A few years ago wafers of 2-inch diameter were commonplace, and 3-inch diameter wafers were considered large. Today much of the device fabrication is done with 4-inch diameter wafers and widespread use of 5-inch wafers within a very few years is foreseen. The reductions in device size, coupled with the increased size of wafers, have served to greatly increase the economic value of individual wafers, and thus the need to process and metallize such wafers in an improved manner.
Most semiconductor and microcircuit fabrication techniques require deposition of metallic coatings of high quality upon the semiconductor wafer upon which the microcircuits are defined. Whether a coating is of "high" quality will of course ultimately be determined by the degree of satisfaction with the ultimate yield of microcircuit devices from the wafer, and their utility, for example, as meeting the higher military or industrial standards, or the lesser consumer and hobbyist standards. Although therefore difficult to quantify, it is generally agreed that metallization quality, and thus ultimate quality and quantity of yield, will in turn be a function of the following factors: uniformity of coverage upon the uppermost and main planar surface of the wafer ("planar coverage"); contamination levels incorporated into the final coating; defect level caused by debris; symmetry and homogeneity, or in other words freedom from "layering" and the manner of distribution of contaminant levels in the film; the degree of reproducibility and control, especially of temperatures during coating deposition; and step coverage, that is, the continuity and evenness of the coating across not only the main plane of the surface, but also the sides and bottoms of such features within the surface as steps, grooves, depressions, and raised portions which define the microcircuits.
Some of these characteristics are harder to achieve or are more critical than others, or have been thought to require definite specialized processing steps to achieve. For example, because of the constraints of geometry, step coverage has been a particularly difficult requirement to fulfill. The sidewalls of the steps and grooves are generally perpendicular to the uppermost surface of the main plane of the wafer, and may face both inwardly and outwardly of the wafer center. Covering of such perpendicular surfaces, particularly the outer-facing ones, while at the same time covering the planar surfaces, is obviously an especially difficult problem, yet such "step coverage" is of particular importance in determining the quality of the metallization overall. It has generally been thought that in order to achieve the required uniformity of planar surface coverage as well as adequate step coverage, relative motion between the wafers and the deposition source during coating deposition is necessary. However, such motion carries with it certain disadvantages, especially the heightened possibility of generation of debris, as by dislodgement of deposits of coating material on various internal structures of the apparatus due to the motion, a heightened possibility of mechanical shock and vibration damage to the wafer, and the buildup of deposition on the wafers in a nonsymmetrical and inhomogeneous fashion, as will be further explained below. Naturally, contamination level will depend on the maintenance of the quality of the vacuum environment during deposition and the concentration of contaminants relative to the speed of deposition. Thus the adequacy of "outgassing", or the evacuation of gas and vapors from the wafer and accompanying wafer supports which are introduced into the coating chamber will also be important.
The manner in which the prior art has attempted to achieve one or more of the above characteristics, and the attendant difficulties and trade-offs involved in achieving the above criteria of coating quality, may be best appreciated by considering the two main types of vacuum deposition systems which are in current use for metallizing wafers: batch and load lock. A typical batch system comprises a pumping station, an evacuable bell jar, an isolation valve between the pumping station and the bell jar, heat lamps, one or more deposition sources, and planetary fixtures which hold the semiconductor wafers and rotate them above the deposition source or sources. At the start of a deposition cycle the isolation valve is closed and the bell jar is open. Wafers are loaded manually from cassettes into the planetary fixtures (a load of 75 wafers of 3-inch diameter is typical). The planetary fixtures are then mounted in the bell jar, the bell jar closed, and the system evacuated. After a prescribed base pressure is reached, the wafers are further outgassed through the application of radiant energy from the heat lamps. In some cases the wafers are sputter-etch cleaned prior to the start of deposition. A typical coating is aluminum or an aluminum alloy sputtered onto the wafer to provide interconnect metallization. In order to achieve the required coating uniformity and step coverage, relative motion is provided by rotation of the planetary fixtures. After deposition, the wafer and system are allowed to cool, the isolation valve is closed, the bell jar vented to atmosphere, the bell jar opened, and the planetary fixtures are removed and unloaded manually into cassettes. This completes a typical cycle, which takes approximately 1 hour.
Although such batch systems are in widespread use in production today for metallizing semiconductor wafers, certain of their characteristics pose limitations and disadvantages. For one, the entire relatively large batch of wafers is inherently "at risk" of a partial or total loss during the deposition cycle. The manual loading of wafers from cassettes into planetary fixtures provides ample opportunity for contamination and breakage. Air exposure of the entire system inside the bell jar for loading and unloading leads to possible contamination and adds a very large outgassing load for the vacuum pumps to contend with (the outgassing area ascribable to the wafers alone is typically less than ten percent (10%) of the total air-exposed area which must be outgassed). Long deposition throw distances (typically 6 to 14 inches) from the source are needed to obtain the large area coverage for the many wafers to be coated within the batch system. This leads to low deposition rates (typically 600 angstroms per minute for sputter deposition source), which make the films more susceptible to poisoning by reaction with background gases, and thus more sensitive to the quality of the evacuated environment. Outgassing of the wafers and the air-exposed areas of the system is accelerated by application of radiant energy from heat lamps, but since the wafers are in uncertain thermal contact with the planetary fixtures, their temperatures are also uncertain. Moreover, the heating source normally cannot be operated during sputter deposition, so that the wafers cool in an uncontrolled fashion from the temperature attained during preheat. Lack of control of wafer temperature during deposition limits certain aspects of film characteristics which can be reliably and reproducibly attained. Of course the mechanical motion of the planetary fixtures for achieving uniformity and step coverage can dislodge particles of coating material deposited elsewhere within the system other than on the wafers, which in turn can cause debris to become attached to the wafers, in turn reducing yield of good devices.
A typical load lock system comprises a pumping station, an evacuable processing chamber, an isolation valve between the pumping station and the processing chamber, a heating station, a deposition source, a load lock, and a platen transport system. At the start of a deposition cycle, wafers are loaded manually from a cassette into a metal platen (a 12-inch by 12-inch platen size is typical), which then acts as a carrier for the wafers during their journey through the load lock and processing chamber. After introduction through the load lock into the processing chamber, the platens and wafers are transported to the heating station, where they are further outgassed by the application of radiant energy. Additional cleaning of the wafers by means of sputter etch may also be performed at the heating station. Metal film deposition is accomplished by translating the platen and wafers relatively slowly past the deposition source, which may be a planar magnetron type of sputtering source with a rectangular erosion pattern, with the long dimension of the erosion pattern being greater than the platen width. Relatively high deposition rates (10,000 angstroms per minute) are achieved by moving the platen past the sputter source over a path which passes the wafers within several inches of the sputter source. After deposition, the platen and wafer are returned to the load lock where they pass from the processing chamber back to atmosphere. The wafers are then unloaded manually back into a cassette. This completes a typical cycle, which takes typically 10 to 15 minutes. In another type of load lock system, the wafers are mounted on an annular plate which rotates past the deposition source. Each wafer makes multiple passes below the deposition source until a film of sufficient thickness is built up.
The above load lock systems overcome some of the disadvantages of batch systems, but not all. Of primary importance is the fact that the use of a load lock allows wafers on a platen to be introduced into and removed from the processing chamber without allowing the processing chamber pressure to rise to atmospheric. This greatly reduces the amount of air-exposed surface that must be outgassed prior to deposition. While the processing chamber does need to be opened to atmosphere periodically (for cleaning and for replacing deposition targets), the frequency of such air exposure is much less than with batch systems.
Another important factor is that the size of the wafer load which is "at risk", that is, subject to being rejected due to a defect or failure of the process, is significantly smaller in the load lock system (16 3-inch wafers in the first load lock system, compared with 75 3-inch wafers in the batch system in the above example). Because the number of wafers per load is much smaller with the load lock system, it is not necessary to employ the long deposition throw distances required with batch systems. Higher deposition rates are therefore attainable by closer coupling between wafer and source.
Despite the advantages afforded by load lock systems, many disadvantages and shortcomings still remain. In both batch and load lock systems, wafers are typically transferred manually between platen and cassette, with attendant risks of contamination and breakage. Although the use of the load lock avoids exposure of the processing chamber to the atmosphere, the platen which carries the wafers is air-exposed on each load and unload cycle. Thus its surfaces must also be outgassed, which raises the total outgassing load well beyond that of only the wafers themselves. In addition, sputtered deposits that build up on the platen become stressed due to repeated mechanical shock and air exposure, leading to flaking and debris generation. As with batch systems, wafers are still in uncertain thermal contact with their carrier. Controls over wafer temperature during outgassing and during deposition remain inadequate. The metal films are put down on the wafers in a nonsymmetrical fashion, since the film deposited on the wafer builds up in different ways depending upon its location on the platen, i.e., whether the wafer is outboard, inboard, approaching the source, or moving away from the source. Translation of the platen during deposition to provide uniformity and step coverage heightens the risk of generation of debris and flakes, and thus contamination of the wafers. In certain load lock systems, symmetry and homogeneity are further compromised by causing the wafer to make multiple passes below the deposition source. Thus the metal film is deposited in a "layered" fashion because the deposition rate tapers off to almost nothing when the wafers are rotating in a region remote from the deposition source. The low rates of deposition in such regions heighten the risk of contamination due to incorporation of background gases into growing film, and non-uniformities in the distribution of any contaminants which may be present result from the non-uniformities in deposition rate.
Even though a much smaller number of wafers is being processed at any one time in load lock systems as compared with batch systems, a significant number of wafers still remains "at risk". From this point of view, it would be best to process wafers individually on a serial continuous basis, but the time needed for adequate pumpdown of the load lock during loading and unloading, and for wafer outgassing and the outgassing of wafer supports, coupled with the time needed to coat a wafer individually in an adequate manner, has rendered the concept of such individual processing impractical until now as compared to batch systems or load lock systems handling a plurality of wafers with each load. It would also be much better from the viewpoint of prevention of debris generation and consequent reduction in yield of good microcircuit devices, as well as lessening the risk of abrasion and mechanical shock and vibration, to hold wafers stationary during coating deposition. However, as we have seen, this has been considered inconsistent with the need to obtain adequate deposition uniformity and step coverage, since this normally requires establishing relative motion between the source and wafer. Further, there has been no basis for expecting greater control over reproducibility and temperature of the coating process in a individual wafer processing system as opposed to a batch or load lock system coating a plurality of wafers with each load.
Accordingly, an object of the invention is to provide apparatus for rapidly coating wafers individually with a higher quality coating than possible previously.
A related object of the invention is to provide apparatus for depositing metallization layers of superior quality with respect to the aggregate considerations of step coverage, uniformity, symmetry and homogeneity, contamination level, debris damage, and reproducibility.
Also an object of the invention is to provide apparatus for rapidly coating wafers individually with improved step coverage and good uniformity.
Another related object of the invention is to provide an improved load lock system for metallizing wafers individually yet at a high rate.
Yet another object of the invention is to provide an improved load lock system for metallizing semiconductor wafers individually on a production-line basis with enhanced quality, including uniformity and step coverage.
A related object is to provide a system for coating wafers which reduces the number of wafers at risk at any one time due to processing.
Another related object is to provide a system for metallization or other vacuum processing of wafers individually on a serial continuous basis, with a plurality of work stations operating simultaneously on individual wafers.
Also a related object is to reduce the outgassing load and minimize disturbance to the evacuated coating environment due to introduction of wafers into a load lock system for coating.
Yet another object of the invention is to improve the yield of microcircuit devices subsequently derived from the wafer by reducing generation of debris and the probability of damage from abrasion and incorporation of contaminants.
Yet another object is to provide a load lock type system which accomplishes transport between various work stations and into and from the vacuum regions without the use of platen-like supports for the wafers.
Also a related object of the invention is to provide a load lock type system as above which does not use platen-like wafer supports, in which loading and unloading are effected of certain wafers while yet others are being processed.
A further related object is to provide a system as above which is compatible with automatic wafer handling from cassettes.
Also a related object is to provide improved control over wafers, especially their temperature, throughout the processing thereof.
Yet a further object of the invention is to provide a system for production-line use in which reliability, maintainability, and ease of use are improved.